This invention relates generally to techniques for intermingling a disinfectant with wastewater to be treated to kill pathogens therein, and more particularly to a gravity flow disinfectant contactor having minimal energy requirements adapted to enhance contact between the disinfectant and the water to effect a rapid and efficient disinfection activity.
Chlorination is widely used to purify water supplies. In practice, chlorine is introduced at a selected point in the water supply system, flow then taking place into a tank or through a region of flow which is sufficient for the chlorine to act effectively on the contaminants present in the water to produce a disinfecting action. The amount of chlorine added to the water is referred to as the "dosage", and is usually expressed as milligrams per liter (mg/l) or parts per million (ppm). The amount of chlorine used up or consumed by bacteria, algae, organic compounds and some inorganic substances, such as iron or manganese, is designated as the "demand."
Since many of the reactions with chlorine are not instantaneous, but require time to reach completion, chlorine demand is time-dependent. The amount of chlorine remaining in the water at the time of mesurement is referred to as the "residual." Residual is therefore determined by the demand substracted from the dosage. Inasmuch chlorine demand is time-dependent, this dependency is likewise true of chlorine residual.
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acids is formed. The hydrochloric acid always completely dissociates into hydrogen and chloride ions, whereas the hypochlorous acid only partially dissociates into hydrogen and hypochlorite ions as a function of the pH of the water. In either the hypochlorous or hypochlorite form, chlorine is called "free chlorine residual." Free chlorine residual has a highly effective killing power toward bacteria.
Should the chlorinated water contain ammonia or certain amino (nitrogen-based) compounds, as is invariably the case with sewage, then additional compounds, called chloramines, are created. Chloramines may occur almost instanteously, depending mainly on water pH. Though several reactions are possible between hypochlorous acid and ammonia, chloramines collectively are referred to as "combined chlorine residual." This combined chlorine residual has a much lower bactericidal effect than free chlorine residual.
Domestic wastewater is typically high in ammonia, the ammonia resulting primarily from hydrolysis of urea. Almost all of the inorganic nitrogen formed in solutions that enter a waste treatment plant is normally in the least oxidized, ammonia form. In conventional secondary waste treatment, a portion of the ammonia will be completely nitrified to nitrite, some ammonia will be only partially nitrified to nitrite, and a portion will remain as ammonia.
When sufficiently high chlorine dosages are applied to waters containing ammonia, different reactions will occur, resulting in the destruction of the ammonia and the formation of free chlorine residual. Thus, for water containing a known amount of ammonia, if one starts with a chlorine dosage which is low, chloramines will be formed resulting in a combined chlorine residual whose bactericidal effect is relatively weak.
As the dosage is raised, the amount of combined chlorine residual produced also increases, until a peak is reached when all of the free ammonia is used up in the formation of chloramine. And as the dosage is elevated beyond the level at which the combined chlorine residual peaks, destruction of the chloramines, which are unstable, takes place until a breakpoint is reached indicating that chloramine destruction is at its maximum. At breakpoint, the first persistent appearance of free chlorine occurs. Thus by using a chlorine dosage sufficient to attain the breakpoint state, one is able to get rid of virtually all ammonia and most of the chloramines.
The virtues of chlorination have long been appreciated, but it is only recently that the hazards involved in excessive chlorination have been publicly recognized. In studies carried out in the chlorinated water supply of the City of New Orleans, it was found that the levels of chlorination were such as to release carcinogenic agents dangerous to the community. The results of this study are reported in the article by R. A. Harris, "The Implication of Cancer Causing Substances in Mississippi River Water," published by the Environmental Defense Fund, Washington, D.C., Nov. 6, 1974.
Shortly after this study appeared, Public Law 93-523 went into effect authorizing the ERA administrator to conduct a comprehensive study of public water supplies "to determine the nature, extent, sources of, and means of control of contamination by chemicals or other substances suspected of being carcinogens."
Subsequently, Jolley ("Chlorine-containing Organic Constituents in Chlorinated Effluents"--Journal of the Water Pollution Control Fed., 47:601--618 (1975) reported the presence of forty-four chloro-organic compounds in a chlorinated secondarywastewater effluent.
Many applications exist for chlorine in wastewater treatment facilities, such as for odor control of raw sewage and the control of hydrogen sulfide in sewers, but its most universal application lies in wastewater treatment facilities for the terminal disinfection of the treated plant effluent just before the effluent is discharged.
The formation of compounds suspected of being carcinogenic as a result of the reaction of chlorine with hydrocarbons in wastewater is by no means the only unwanted side effect caused by the traditional disinfection process, for chlorine residuals in wastewater give rise to an environment that is toxic to aquatic organisms. Though chlorine is a highly effective biocide for undersirable organisms, it is also deadly to fish and other forms of aquatic life and therefore has a deleterious impact on fresh water eco-systems.
In general, wastewater disinfection practice has heretofore largely disregarded these unwanted side effects, for this practice focused on the two factors thought to be of greatest significance in attaining adequate disinfection; namely, the residual of the disinfectant and its contact time with the sewage. This practice has brought about the use of massive doses of disinfectant in long serpentine channels serving to prolong contact time. While this produced the desired degree of disinfection, it also aggravated unwanted side effects.
In order to obtain adequate disinfection with minimal unwanted side effects, the now-recognized goal is to carry out rapid, intimate mixing of the chlorine solution with the wastewater stream in the shortest possible period.
Thus one of the most important facets of a good chlorinator installation is the chlorine solution diffuser which injects the chlorine into the pipe or channel carrying the potable water or wastewater to be disinfected. It has heretofore been the practice to introduce the chlorine solution into the central region of the pipe or channel, for it was believed that this would cause the chlorine to mix with the water in the shortest possible time.
In one prior diffuser system based on this approach, use is made of a small pipeline serving as a mixing device, the chlorine solution being injected through a tube extending to the centerline of the pipe to discharge the solution at right angle to the water flow direction. In this prior arrangement, the water stream flowing through the pipe deflects the chlorine injected therein to cause it to intermingle downstream with the water to produce the necessary mixing action.
In another known diffuser installation designed for larger closed conduits, an array of radially-arranged injector tubes is provided to introduce chlorine solutions into the central region of the conduit, again at right angles to the direction of water flow. For large open channels, it is known to support a chlorine solution manifold line above the channel. Injection tubes are suspended from this manifold line and extend into the channel for injecting the disinfectant into the channel at the central region therein.
In still another known arrangement designed for pipes larger than 36 inches in diameter, a diffuser tube having a series of jet apertures therein is supported centrally across the pipe along a diametrical axis to produce a row of jets above and below the tube at right angles to the water flow.
The difficulty with the above-described prior diffuser arrangements is that the chlorine solution in all instances is discharged into the flowing water at the central zone of the pipe or channel so that the disinfectant discharge takes place within a confined region. In order to effect thorough mixing of the chlorine solution with the water being treated, it is essential that the chlorine be dispersed throughout the body of the water. With prior arrangements such disposal does not occur at the point of injection but much further downstream in the pipe or channel, as a consequence of which, the mixing action fails to take place in the shortest possible time.
Ideally, the mixing time for the chlorine solution should be a fraction of a second. With a view to overcoming the limitations of conventional diffuser arrangements, a jet disinfection technique has been developed to accelerate the mixing activity. This technique is described in the Penberthy Jet Disinfection Technical Bulletin published in 1977 by the Pentech Division of Houdaille Industries, Inc. of Cedar Falls, Iowa, U.S. Pat. No. 4,019,983)
In this jet disinfection technique, the influent to be treated is pumped into a jet nozzle to which a chlorine supply is coupled, the nozzle projecting the influent into a reactor tube into which the chlorine is drawn by induced vacuum. Because of the highly turbulent field existing within the reaction tube, the disinfectant is thoroughly dispersed throughout the entire effluent flow and for an instant subjects the bacteria and viruses to an acutely toxic environment.
In a typical jet disinfection installation, a sealed baffle is placed across the wastewater channel to direct all channel flow through a plurality of reactor tubes. A portion of the incoming wastewater flow is internally pumped into the jet nozzle associated with each reactor tube, the chlorine being carried into the jet by induced vacuum. Thus, in addition to the external pump requirements for the channel, each jet nozzle assembly requires its own internal pump, thereby adding substantially to the overall cost of installing and operating the system, particularly with respect to power consumption.